A variety of electrolytic baths for electroplating cadmium on metallic substrates have been employed in the past. These baths typically utilize sulfates and cyanides as the primary electrolytes therein. Cyanide baths, while proving effective in a commercial sense, have significant objectionable features, not the least of which are toxicity of the bath components and difficulty of waste disposal due to the cyanide ion therein.
Heretofore available sulfate-based baths, while overcoming many of the objectionable features of the cyanide baths, have been found to exhibit relatively low throwing power and poor efficiency at low current densities resulting in an increase of metal in the bath upon continued bath operation. Furthermore, most of the sulfate-based baths contain such components as ammonium ions and chelating agents. Such components, because of their ability to complex with heavy metal ions, significantly increase the difficulty of eliminating heavy metals from spent baths.
A chloride-based cadmium bath which has overcome many of the problems of the sulfate baths, and has been popular commercially, is disclosed in U.S. Pat. No. 3,577,327. However, this bath also contains ammonium ions and chelating agents, e.g. ethylenediaminetetracetic acid (EDTA), nitrilotriacetic acid (NTA) and citric acid. Thus, removal of heavy metals from spent baths is similarly difficult and expensive waste treatment procedures and equipment must be utilized.
In U.S. Pat. No. 3,826,722 there is disclosed an acid cadmium sulfate bath which does not contain ammonium ions or chelating agents therein. However, the effectiveness of the bath is dependent upon the inclusion of a thiosemicarbazone constituent. It is believed that this compound is somewhat unstable in strong acid solutions, and upon continued use in an electrolytic bath may result in cadmium deposits which are dull in appearance over an appreciable cathode current density range. Sulfur contamination of the bath from breakdown of this thiosemicarbazone compound on extended use can also prevent bright-dipping of the work piece subsequent to cadmium deposition.